1. Field of the Invention
The present invention relates to branching filters. In particular, the present invention relates to branching filters that include a ladder elastic wave filter connected between an antenna terminal and a transmission signal terminal and a longitudinally-coupled-resonator-type elastic wave filter that is connected between the antenna terminal and first and second reception balanced signal terminals and that has a balanced-unbalanced transformation function.
2. Description of the Related Art
In recent years, elastic wave branching filters, which utilize elastic waves, such as surface acoustic waves or boundary acoustic waves, have been widely used as branching filters that separate transmission signals and reception signals transmitted from and received by an antenna.
For example, in PCT International Publication No. WO 2007/116760 A1, a duplexer is described that includes a ladder elastic wave filter having a high electric power handling capability as a transmission filter and a longitudinally-coupled-resonator-type elastic wave filter having a balanced-unbalanced transformation function as a reception filter.
In the duplexer described in PCT International Publication No. WO 2007/116760 A1 that includes a longitudinally-coupled-resonator-type elastic wave filter having a balanced-unbalanced transformation function as a reception filter or a transmission filter (hereafter “balanced duplexer”), excellent balance characteristics are required between the first and second balanced signal terminals. Consequently, it has been necessary that IDT electrodes, wiring and other elements be arranged such that the balance characteristics between the first and second balanced signal terminals are excellent and, as a result, there have been severe constraints on the structure of electrodes in balanced duplexers. Therefore, size reduction has been difficult to achieve in balanced duplexers.